1. Field of the Invention
This invention relates to excimer laser source systems, and more particularly relates to excimer laser source systems having output radiation that is stabilized in its center frequency, narrowed in its wavelength spread, and suitable for high resolution projection lithography systems.
2. Description of the Prior Art
Lithography systems are extensively used in the production of integrated circuit chips and electronic circuit boards. Such systems typically include a primary light source such as a high intensity lamp or a laser, mask and substrate positioning systems, a projection system to illuminate and image the pattern present on the mask onto the substrate, and a control system. The intent typically is to illuminate a wafer coated with a layer of a photosensitive material so as to produce the desired circuit pattern, which later will be metallized or otherwise activated during further processing. Illumination may be ultraviolet light or visible light or other radiation. The desire is to illuminate the target regions selectively so as to activate a particular pattern. Integrated circuit chips typically undergo numerous illumination steps and physical treatment steps during production.
As the demand for chips with ever greater memory and processing capability increases, the individual bits on the chips get smaller in dimensions. This requires that the lithography equipment used for imaging these patterns have higher and higher resolution. Simultaneously, the larger physical size of the chips demands that the higher resolution be achieved over a larger image field. The requirement of higher resolution has led to the use of shorter wavelengths and projection lenses of higher numerical apertures. The demand for larger image field sizes has resulted in designs of projection lenses of increasing complexity.
The move towards lithography using radiation of shorter wavelengths has progressed from conventional ultraviolet (UV) wavelengths (435 nm region) to mid-UV wavelengths (365 nm region) to deep UV wavelengths (250 nm region). Among deep UV sources for lithography, excimer lasers have been found to be the most attractive due to their high power output as well as desirable spatial and spectral characteristics. Lithography in the deep UV also requires that suitable optical materials be available for fabrication of the projection lenses. The design of a high-resolution projection lens assembly requires the use of different optical materials with different refractive indices in order to achieve the required resolution over the desired image field size. In the conventional UV and mid-UV regions, the lens designer may choose from a number of available glasses of different refractive indices. However, the choice of suitable optical lens materials in the 250 nm region is severely limited. Only fused quartz possesses the optical and mechanical properties required of a material to qualify it as suitable for fabrication of projection lens elements for deep UV lithography. With only one optical material to work with, the designer of a deep UV projection lens must therefore be constrained to use a radiation source of very narrow bandwidth --on the order of a few thousandths of a nm --in order to achieve distortion-free imaging over a suitable field size. Along with the narrow bandwidth, a high degree of stability of the center wavelength of the laser also becomes an important requirement. Typical mercury arc lamps, with linewidths of several nm, and conventional excimer lasers, with linewidths of several tenths of a nm, are both unsuitable for use with deep UV all-quartz projection lenses. These limitations have led to the development of various techniques for narrowing the spectral bandwidth of an excimer laser.
An approach used in the prior art is to employ a parallel-plate etalon placed inside the optical cavity of the excimer laser. The etalon acts as a frequencyselective element by providing high transmission over only a narrow wavelength band and introducing sufficient loss at other wavelengths; this forces the laser to lase over the narrow transmission band of the etalon. A variation of the above approach uses two intra-cavity etalons. In another variation --a further improvement of the above approach --a third etalon is placed outside the laser cavity to monitor the laser wavelength and control the two intra-cavity etalons. The three-etalon method enables one to obtain with an excimer laser a spectral bandwidth of 0.006 nm (.+-.0.003 nm) and a wavelength stability of .+-.0.001 nm. Other approaches in the prior art have used prisms and diffraction gratings, instead of etalons, as the frequency-selective elements nn the laser cavity. However, since the basic technique in these methods has been to use intra-cavity frequency-tuning optical components, the results have been similar to the etalon method.